Soil testing for corn nitrogen recommendations

Quick facts

Over-applying nitrogen (N) to corn causes concern due to the environmental consequences of unused N. There’s also a significant economic advantage to applying the correct rate of N fertilizer.

Excess, unused N can be lost from the soil system via denitrification and/or leaching, get tied up in the soil organic N pool or stay in the crop’s rooting zone as residual N.

Research indicates that measuring residual, available N through a soil test can refine earlier N recommendations for corn. This soil N test involves collecting 0- to 2-foot soil samples in the spring before planting and analyzing samples for nitrate-N. The quantity of nitrate-N found is then used to calculate a residual N credit.

Soil N test research

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Figure 1: Map of Minnesota indicating areas that use a soil nitrate test (shaded area in western Minnesota) and counties where the newer soil N test research was conducted (X's)

In the 1980s, increased interest in economic and environmental concerns led researchers to revisit the concept of including a soil N test to refine fertilizer N recommendations for corn in Minnesota’s humid regions.

A major research study conducted from 1989 through 1992 provided data from 59 sites in 19 counties (see Figure 1). This led to the development of a new soil N test throughout Minnesota.

Research was conducted on a variety of soil types, primarily on farmers’ fields. A host of sampling depths, times and analysis parameters were evaluated in countless combinations. Grain yield response to N was best indicated by soil nitrate-N measured from a two-foot sampling depth.

The direct correlation between yield response to N and soil N test wasn’t as strong as desired, but subsequent calculations led to a strong correlation between soil nitrate-N at preplant time and unaccounted N using the traditional recommendations (i.e., without using a soil test).

The unaccounted N was deemed to be residual N. You can see this relationship for one set of data in Figure 2, which is the basis for the residual N credit in Figure 3.

Figure 2: Linear relationship between 0 and 2 feet of nitrate-N concentrations measured before planting and residual N credit for a subset of the dataset

Figure 3: Soil N credit amounts as a delineated, adjusted function of the preplant nitrate-N concentrations. At 19 parts per million (ppm) or greater nitrate-N, no fertilizer N would be recommended.

Using the soil N test

This soil N testing option, which accounts for residual N, isn’t appropriate for all fields and conditions.

Use Figure 4 to help decide which fields you may need to test for residual nitrate-N. This flowchart uses factors such as previous crop, manure history and a knowledge of previous rainfall.

Figure 4: Flowchart decision-aid for determining the probability of having significant residual N in the soil.

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Although the western tier of Minnesota has long had a process for factoring soil N test information into corn N recommendations, western Minnesota producers can use this newer procedure too.

Slight differences in N recommendations may appear when comparing the two procedures for a given site due to mathematical rounding differences inherent in each procedure.

First, evaluate whether conditions exist for residual N to accumulate. A soil N test isn’t necessary for all Minnesota acres to be planted to corn.

Research evaluating N recommendations made with and without the preplant soil N test identified some conditions where the new preplant soil nitrate-N test has the greatest utility and impact.

Consider previous crop, soil texture, previous rainfall and manure history to determine which fields are most likely to benefit from the N test, as these factors significantly affect the accumulation of residual N.

Previous crop

A crop rotation that has corn following corn generally provides the greatest potential for significant residual N accumulation. This is probably due to the amount of fertilizer N added to the previous corn crop, the fact that corn doesn't scavenge excess N from the soil – in contrast to a legume – and frequent manure additions made to continuous corn fields.

In contrast, when soybean is the previous crop, much less residual N has been measured. Soybeans will use residual N in the soil in addition to the N that it symbiotically fixes within its nodules.

Soil texture

The new soil N test works best on medium- and fine-textured soils derived from loess or glacial till. Using the proposed soil N test on coarse-textured soils derived from glacial outwash doesn’t appear to be promising. Outwash sites consistently had low soil N test values in the study, and you can’t expect to measure significant residual N in these sandy soils.

Rainfall

The amount of residual N in the soil also depends on the rainfall received the previous year. In a year following a widespread drought (e.g., 1989), the majority of fields will have significant residual N. However, following relatively wet years, such as the early 1990s, you can expect little residual N.

Manure applications

Previous manure applications to a field generally increase residual N in the soil. While the perception is that the most recent manure applications (year of or year after application) would best be reflected by the soil N test, research experience indicates that the residual effects – two, three or four years after application – are best-quantified by this new test.

You can make nitrogen fertilizer recommendations for corn with or without the new soil N test.

The University of Minnesota’s previous N recommendations, which assume minimal residual N, are still the starting point for all recommendations. This is because the soil N test is used to estimate residual, available N.

Here’s a suggested five-step process for determining the N recommendation:

Determine N recommendation using yield goal, previous crop and organic matter content for the specific field. Use Table 1 (Table 1 in Fertilizing Corn in Minnesota) to do this. The prescribed rate recommendation assumes you’ll follow best management practices for the specific conditions.

Determine whether conditions are such that residual N may be appreciable. Figure 4, which includes factors such as previous crop, manure history and previous fall rainfall can provide insight as to the applicability of testing for N. If conditions are such that the probability of residual N is small and soil testing for N isn’t recommended, use the N recommendation derived in Step 1.

If soil N testing is recommended, collect a preplant, 0- to 2-foot soil sample, taking enough soil cores so the sample is representative of the entire field. Then, send the sample to a laboratory for nitrate-N analysis.

Determine residual N credit based on the measured soil nitrate-N concentrations. Use Figure 3 to determine this credit.

Calculate the final N recommendation by subtracting the residual N credit (Step 4) from the previously determined N recommendation (Step 1). This fertilizer N amount can then be applied either preplant and/or as a sidedress application.

Don’t use this soil N test if you applied commercial fertilizer the previous fall. The variability in the degree of N conversion to nitrate-N before the spring makes this test meaningless in these situations.

Manure additions to soil can significantly increase soil nitrate-N concentrations. However, the amount of nitrate-N at any given time (i.e., spring preplant) is a function of the time of manure application and method of application.

The following scenarios may help you understand how to most effectively use the new soil N test in combination with manure management to determine N recommendations.

When a soil N test isn’t used

When a soil N test isn’t used, credit manure N. Do this by calculating the amount of N that will be available (manure rate x manure N content x N availability) and subtracting this amount from the N recommendation determined based on yield, organic matter and previous crop.

Spring manure applications

If you’re applying manure in the spring before corn planting, collect the soil samples for N measurement before manure application. Determine the N recommendation and then separately calculate the manure N credit and subtract this amount from the previous N recommendation derived from the soil N test.

Winter manure applications

If you applied manure during the winter months, it’s assumed that the majority of the inorganic N would be lost via volatilization. So, you can use the soil N test to determine an N recommendation.

However, you should subtract the manure N credit – although less than that of spring-applied manure – from this amount. Keep in mind that spring sampling can occur anytime after the frost is out of the ground.

Fall manure applications

Oct. 1 to Dec. 1

If you applied manure the previous fall between Oct. 1 and Dec. 1, you can’t use the soil N test or the manure N worksheet credit system.

Not taking a soil N test and just using the standard manure N crediting system may result in high fertilizer recommendations if significant residual N was present before the manure was applied.

In contrast, only using the spring soil N test to credit fall-applied manure will underestimate the credit due to the manure. This is because all of the available manure N will not have converted to nitrate-N by this time.

If manure had been applied in this time frame and the previous crop was soybeans, base the N recommendation on the standard manure crediting system. If corn was the previous crop, base the N recommendation on the soil N test, knowing that the soil test may underestimate the available N coming from the manure.

Before Oct. 1

If you applied manure the previous fall before Oct. 1, the soil N test will probably pick up most of the N from manure that will be available during the growing season. The amount of N released from manure's organic N fraction after taking the preplant sample will be minor compared to the accuracy of the N recommendation provided by the soil testing option.

The test does an excellent job accounting for residual N where manure had been applied 11 months or more before collecting the preplant soil N test samples. This test should work particularly well in situations with a long history of manure applications.

Other procedures for N recommendations

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Soil N testing has been a management tool for western Minnesota (see Figure 1) for many years. It’s recommended to take the late fall or spring preplant N soil test and then subtract the measured amount of nitrate-N according to the following formula.

NRec = (1.2)(YG) - STN(0-24in.) - NPC

Where:

NRec = Amount of fertilizer N needed, in pounds per acre

YG = Realistic yield goal, in bushels per acre

STN(0-24 in.) = amount of nitrate-nitrogen

(NO3 - N) measured by using the soil nitrate test, in pounds per acre

NPC = Nitrogen credits, if any, for previous crops in the rotation, in pounds per acre

In the rest of the state, corn nitrogen recommendations have traditionally been made using a much different approach.

The approach specific to western Minnesota isn’t appropriate for eastern and central Minnesota, due to the higher rainfall it receives. For N recommendations in regions with higher rainfall, use Table 1. The table accounts for organic matter levels, previous crop and expected yield.

The well-drained silt loam soils in southeastern Minnesota receive the N recommendations listed for soils with a medium and high organic matter level. All irrigated soils are included in the medium and high organic matter category.

For small grains, use the N credits only if stubble is tilled after harvest. If there was no tillage, use recommendations for crops in Group 2.

Table 1: Nitrogen recommendations for corn where the soil nitrate test isn’t used

Crop grown last year

Organic* matter level

Yield goal: 70-90 bushels per acre (bu/a)

Yield goal: 91-110 bushels per acre (bu/a)

Yield goal: 111-130 bushels per acre (bu/a)

Yield goal: 131-150 bushels per acre (bu/a)

Yield goal: 151-170 bushels per acre (bu/a)

Yield goal: 171-190 bushels per acre (bu/a)

Yield goal: 191+ bushels per acre (bu/a)

Alfalfa (4+ plants per square foot), non-harvested sweetclover

Low (less than 3%)

0 lbs. per acre

0 lbs. per acre

0 lbs. per acre

0 lbs. per acre

30 lbs. per acre

50 lbs. per acre

70 lbs. per acre

Alfalfa (4+ plants per square foot), non-harvested sweetclover

Medium and high (3% or more)

0 lbs. per acre

0 lbs. per acre

0 lbs. per acre

0 lbs. per acre

0 lbs. per acre

20 lbs. per acre

40 lbs. per acre

Soybeans, small grains, alfalfa (1 or fewer plants per square foot)

Low (less than 3%)

20* lbs. per acre

50* lbs. per acre

80* lbs. per acre

110 lbs. per acre

140 lbs. per acre

160 lbs. per acre

180 lbs. per acre

Soybeans, small grains, alfalfa (1 or fewer plants per square foot)

Medium and high (3% or more)

0 lbs. per acre

30 lbs. per acre

60 lbs. per acre

80 lbs. per acre

110 lbs. per acre

130 lbs. per acre

150 lbs. per acre

Edible beans, field peas, harvested sweetclover

Low (less than 3%)

40 lbs. per acre

70 lbs. per acre

100 lbs. per acre

130 lbs. per acre

160 lbs. per acre

180 lbs. per acre

200 lbs. per acre

Edible beans, field peas, harvested sweetclover

Medium and high (3% or more)

20 lbs. per acre

50 lbs. per acre

80 lbs. per acre

100 lbs. per acre

130 lbs. per acre

150 lbs. per acre

170 lbs. per acre

Group 1 crops (see below for list)

Low (less than 3%)

0 lbs. per acre

15 lbs. per acre

45 lbs. per acre

75 lbs. per acre

105 lbs. per acre

125 lbs. per acre

145 lbs. per acre

Group 1 crops (see below for list)

Medium and high (3% or more)

0 lbs. per acre

0 lbs. per acre

15 lbs. per acre

45 lbs. per acre

75 lbs. per acre

95 lbs. per acre

115 lbs. per acre

Group 2 crops (see below for list)

Low (less than 3%)

60 lbs. per acre

90 lbs. per acre

120 lbs. per acre

150 lbs. per acre

180 lbs. per acre

200 lbs. per acre

220 lbs. per acre

Group 2 crops (see below for list)

Medium and high (3% or more)

40 lbs. per acre

70 lbs. per acre

100 lbs. per acre

120 lbs. per acre

150 lbs. per acre

170 lbs. per acre

190 lbs. per acre

*Increase these recommendations by 20 pounds of N per acre to compensate for less N credit on coarse-textured soils.